Since 1973, the increase in fuel costs has prompted research efforts towards developing more fuel efficient aircraft. One solution would be to reduce the weight of structural components without attendant loss in strength or other desirable properties. Intense research efforts led to the realisation of at least three near-commercial, low density Al-Li alloys; to produced by Alcan in the U.K. and the third by Alcoa in the U.S.A. These three alloys 8090 (sometimes referred to by tradenames as DTDXXXA, Alcan A, or Lital A), 8091 (Alcan B, Lital B, or DTDXXXB) and 2090 (Alcoa B) comprise a new generation of Al-Si alloys. In general, such alloys were developed for aircraft applications where the weight savings effected by using these low-density alloys greatly reduces vehicle fuel costs and also increases performance. Because most aircraft parts are mechanically fastened, the weldability of the Al-Si alloys has received relatively limited attention. If weldable Al-Li alloy variants were available commercially they could potentially be used for many non-aircraft applications, such as, marine hardware, lightweight pressure vessels and the like. Since many pressure vessels are used at low temperatures it would be import and for the structural alloys employed to have good mechanical properties at both room and cryogenic temperatures.
Significant events in the development of aluminum base alloys containing lithium for structural applications were the introduction of the Scleron alloys (Al-Zn-Cu-Li), developed in Germany in the early 1920's; alloy 2020 (Al-Cu-Li-Cd) developed in the United States by Alcoa in the late 1950's; and alloy 01420 (Al-Mg-Li) developed in the USSR in the mid-1960's. Alloys 2020 and 01420 essentially constitute the first generation of Li containing Al alloys used on a commercial scale. Commercial aluminum alloys in the U.S. are sometimes described by four-digit numbers assigned under the standard Aluminum Association designation system which is explained in the "Metals Handbook", Ninth Ed. (American Society for Metals, Metals Park, Ohio, U.S.A.), Vol. 2, pg. 44, (1979).
Aluminum and its alloys have desirable properties such as low cost, good appearance, relatively light weight, fabricability, and corrosion resistance that make them attractive for a wide variety of applications. The aluminum base metal referred to herein is about 99.00% pure with iron and silicon being the major impurities; and where the percentage of aluminum in compositions described herein is not specified it is to be understood that the aluminum makes up the difference between 100% and the sum of the specified elements.
Lithium is the lightest metal found in nature and its addition to aluminum metal is known to significantly reduce density and increase stiffness. Consequently, aluminum-lithium alloys could offer valuable combinations of physical and mechanical properties that would be especially attractive for new technology applications, particularly, in industries such as aircraft and aerospace. Lithium is generally known to produce a series of low density (i.e., light), age hardenable aluminum alloys (Al-Li, Al-Mg-Li, or Al-Cu-Li) but these alloys have been used only to a limited extent because, among other things, they were believed to oxidize excessively during melting, casting and heat treatment (Kirk-Othmer "Encyclopedia of Chemical Technology" 3 Ed., John Wiley (1981) Vol. 2, pg. 169).
One of the early commercial aluminum based systems including lithium is the 01420 family developed by Fridlyander et al. which includes several alloy variants. The 01420 alloys and variants are broadly described in U.K. Patent No. 1,172,738. The alloys disclosed by Fridlyander are said to be high strength, low density and have a modulus of elasticity 15 to 20% higher than standard aluminum alloys, as well as, good corrosion resistance. The ultimate tensile strength claimed for these alloys is 29-39 kg/mm.sup.2 and they are comprised of 5 to 6% Mg; 1.8 to 2.4% Li and one or both of 0.05 to 0.2% Zr and 0.5 to 1.0% Mn, the balance being Al. These alloys are basically of the 5XXX Series-type, i.e., their major alloying element is magnesium, and further include lithium. All percents (%) stated herein are percent weight based on the total weight of the alloy unless otherwise indicated.
Another family of aluminum based alloys including lithium is disclosed in U.K. Patent No. 1,572,587 (assigned to Swiss Aluminum Ltd.) and are said to have a combination of unusually advantageous properties including excellent formability, strength and favorable resistance-weldability which results from the increased electrical resistivity induced by lithium. These alloys are typically of the 5XXX Series-type being composed of 1.0 to 5.0% Mg; up to 1% Mn; up to 0.3% Ti; up to 0.2% V and the balance being Al. A 0.3 to 1.0% lithium component is added to increase electrical resistivity. The lithium is in a super-saturated solid solution in the alloy so that ductility, formability and strength properties are improved and retained at elevated temperatures.
Yet another family of aluminum based alloys that may include lithium are the 2XXX (Aluminum Association system), or aluminum-copper alloys. Such a family of alloys is disclosed in U.S. Pat. No. 2,381,219 (assigned to Aluminum Company of America). These alloys are said to have improved tensile properties because they include substantial amounts of copper and small amounts of lithium and at least one other element selected from the cadmium group consisting of cadmium, mercury, silver, tin, indium and zinc. This reference states that lithium is not known to have any pronounced beneficial effect on the tensile properties, i.e., tensile strength, yield strength, elongation or hardness, when not in combination with an alloying element from the cadmium group and that lithium may even be detrimental to tensile properties.
Presently available high strength aluminum lithium alloys do not have good fusion welding properties as reflected by their low resistance to hot tearing. Hot tearing, in general is believed to result from the inability of the solid-liquid region of the weldment to support the strain imposed by solidification shrinkage. Aluminum-lithium alloys are particularly sensitive to hot tearing because of their high coefficient of thermal expansion and high solidification shrinkage. Compositional modifications that enhance weldability may adversely affect other properties such as strength, ductility, stiffness and/or density.
In view of the foregoing, it would be desirable to provide lightweight, high strength, aluminum-lithium alloys having resistance to hot tearing, (good weldability), resistance to cracking during welding and processing, ductility, stiffness, and low density and/or good mechanical properties at cryogenic temperatures.